Bacterial Fc receptors have been identified by their ability to bind to a site within the constant region of various classes and subclasses of mammalian IgG (Myhre, E. B. and G. Kronvall. 1981. Immunoglobulin specificities of defined types of streptococcal Ig receptors. In: Basic Concepts of Streptococci and Streptococcal Diseases. [Holm, S. E. and P. Christensen, eds.] pp. 209-210, Reed Book, Ltd., Chertsey, Surrey). The Fc region of the IgG antibody molecule is associated with the biological effector properties of the molecule while the antigenic recognition elements are located in the two identical Fab portions of the antibody. Consequently, the interaction of bacterial Fc receptors with constant region determinants on the heavy chain of IgG does not interfere with the ability of the antibody to recognize its antigen; it is this property that makes these receptors so useful as tracers of antibody-antigen interaction (See Boyle, M. D. P. 1984. Applications of bacterial Fc receptors in immunotechnology. Biotechniques 2:334-340.)
To date, five types of bacterial receptors have been described based on the reactivity of whole bacteria with different classes and subclasses of mammalian immunoglobulins. The most extensively characterized receptor is the type I receptor isolated from Staphylococcus aureus, and more commonly designated protein A. The type II receptor is found on a few group A streptococci. The type III receptor is present on many group C streptococci and on some human group G streptococci. The type IV receptor is isolatable from a few bovine group G streptococci, and the type V receptor is found on certain Streptococcus zooepidemicus strains.
Mouse monoclonal antibodies have achieved broad applications in immunodiagnostics and in the therapy of disease. The ability to rapidly screen for and isolate mouse monoclonal antibodies has been greatly aided by the use of protein A. Monoclonal antibodies prepared in rats have also been developed and have certain advantages over similar reagents prepared in the mouse system. Rat immunoglobulins, however, react poorly with known bacterial Fc receptors, and, consequently, these reagents are of little value for isolation and characterization of rat monoclonal antibodies. Thus, an Fc receptor capable of identifying rat antibodies could be of tremendous value for screening hybridoma supernatants for specific rat immunoglobulination and for their isolation.